1. Field of the Invention
The present invention relates to a light-emitting device of gallium nitride compound semiconductor which emits a blue light.
2. Description of the Prior Art
Among the conventional light-emitting diodes which emit a blue light is the gallium nitride compound semiconductor. It attracts attention because of its high luminous efficiency resulting from the direct transition and its ability to emit a blue light, one of the three primary colors of light.
The light-emitting diode of gallium nitride compound semiconductor is made up of a sapphire substrate, an n-layer grown on the substrate from a GaN compound semiconductor of n-type conduction, with or without a buffer layer of aluminum nitride interposed between them, and an i-layer grown on the n-layer from a GaN compound semiconductor which is made i-type by doping with a p-type impurity. (Japanese Patent Laid-open Nos. 119196/1987 and 188977/1988)
It is known that the above-mentioned light-emitting diode will be improved in luminous intensity when the i-layer is provided with an electrode of large area because light emission takes place directly under or near the i-layer.
Much has been reported on the study of crystal growth for light-emitting diodes of GaN compound semiconductors. However, only a little has been reported on the process of producing such light-emitting diodes. This is true particularly of the electrode for the i-layer in a light-emitting diode having a MIS (metal insulator semiconductor) structure. It has been disclosed only in Japanese Patent Laid-open No. 46669/1982, and nothing has so far been discussed about how the electrode for the i-layer is associated with light emission.
The electrode for the i-layer has the layer structure as shown in vertical section in FIG. 8 which is a reproduction from the Japanese patent just given above. Referring to FIG. 8, there is shown a light-emitting diode 60, which has an electrode 67 for the i-layer and an electrode 68 for the n-layer. The electrode 67 is formed from nickel deposited on an aluminum substrate deposited directly on the i-layer. The electrode 68 is also formed from nickel deposited on an aluminum substrate deposited in a hole penetrating the i-layer.
A disadvantage of forming the electrode on aluminum in direct contact with the i-layer is that light is emitted from coarse dots rather than a uniform plane, as shown in FIG. 5(a). The resulting light-emitting diode does not have increased luminous intensity despite its large light-emitting area.